The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Negative-working lithographic printing plates are prepared from negative-working radiation-sensitive compositions that are formed from polymers which crosslink in radiation-exposed areas. A developing solution is used to remove the unexposed portions of the coating to thereby form a negative image.
The most widely used type of negative-working lithographic printing plate comprises a layer of a radiation-sensitive composition applied to an aluminum substrate and commonly includes a subbing layer or interlayer to control the bonding of the radiation-sensitive layer to the substrate. The aluminum substrate is typically provided with an anodized coating formed by anodically oxidizing the aluminum in an aqueous electrolyte solution.
It is well known to prepare negative-working lithographic printing plates utilizing a radiation-sensitive composition which includes a photocrosslinkable polymer containing the photosensitive group: ##STR2## as an integral part of the polymer backbone. (See, for example, U.S. Pat. Nos. 3,030,208, 3,622,320, 3,702,765 and 3,929,489). A typical example of such a photocrosslinkable polymer is the polyester prepared from diethyl p-phenylenediacrylate and 1,4-bis(.beta.-hydroxyethoxy)cyclohexane, which is comprised of recurring units of the formula: ##STR3## This polyester, referred to hereinafter as Polymer A, has been employed for many years in lithographic printing plates which have been extensively used on a commercial basis. These printing plates have typically employed an anodized aluminum substrate which has been formed by electrolytic anodization with an electrolyte comprised of phosphoric acid.
Polyesters in addition to Polymer A which are especially useful in the preparation of lithographic printing plates are those which incorporate ionic moieties derived from monomers such as dimethyl-3,3'-[(sodioimino)disulfonyl]dibenzoate and dimethyl-5-sodiosulfoisophthalate. Polyesters of this type are well known and are described, for example, in U.S. Pat. No. 3,929,489 issued Dec. 30, 1975. A preferred polyester of this type, referred to hereinafter as Polymer B, is poly[1,4-cyclohexylene-bis(oxyethylene)-p-phenylenediacrylate]-co-3,3'-[(s odioimino)disulfonyl]dibenzoate. Another preferred polyester of this type, referred to hereinafter as Polymer C, is poly[1,4-cyclohexylene-bis(oxyethylene)-p-phenylenediacrylate]-co-3,3'-[(s odioimino)disulfonyl]dibenzoate-co-3-hydroxyisophthalate.
Manufacturers of negative-working lithographic printing plates of the type described above typically incorporate a print-out composition in a radiation-sensitive layer of the plate which will produce an optical density difference upon exposure to activating radiation. This enables the customer to see the exposed image on the printing plate before it is processed.
A preferred technique for achieving such print-out is through the use of a print-out composition comprising a leuco form of a dye and a photooxidant. A particularly preferred class of photooxidants are photooxidants which have a photoscissionable nitrogen-oxygen bond, because they exhibit important advantages over other photooxidants, including the ability to provide improved print-out density. Examples of photooxidants having a photoscissionable nitrogen-oxygen bond are the sufonyloxy-N photooxidants described in Altland et al, U.S. Pat. No. 4,425,424, issued Jan. 10, 1984. As explained by Altland et al, the leuco form of the dye has one or more removable hydrogen atoms, the removal of which forms a compound colored differently from the leuco form and the photooxidant, which is alternatively referred to as a photoactivator, is capable of converting the leuco dye to the differently colored form when exposed to the activating radiation. The photooxidants of the Altland et al patent are sulfonyloxy-N compounds of the formula: ##STR4## wherein R is a carbocyclic or heterocyclic ring containing from 5 to 10 ring atoms, and Z represents the non-metallic atoms necessary to complete one or more rings containing from 5 to 17 ring atoms.
Further examples of photooxidants having a photoscissionable nitrogen-oxygen bond include the N,N,O-triacylhydroxylamines of U.S. Pat. No. 3,359,109; the O-acylthiohydroxamates and N-alkoxypyridinethiones of U.S. Pat. No. 4,954,415; the N-alkoxypyridinium salts of U.S. Pat. No. Re. 28,240; the oxycarbonyloxy-substituted pyridinium salts of U.S. Pat. No. 4,886,735; the oxime sulfonate esters of European Patent No. 361,907; and the oxime carboxylate esters of European Patent No. 332,158.
While print-out compositions containing photooxidants which have a photoscissionable nitrogen-oxygen bond provide useful results and are of great commercial importance, there is an urgent need in the art to improve the photo-efficiency of such print-out compositions. The print-out composition must generate sufficient dye density upon exposure to be readily observed without causing serious reduction in the speed of the plate. Thus, the print-out composition is typically employed in as small an amount as feasible in order not to dissipate an excessive amount of the exposure energy needed for the photoimaging components of the printing plate. This tends to result in too faint an image to be readily observable. Improvement in the photo-efficiency of the print-out composition is highly desirable, as it enables the production of a more distinct, and therefore more readily discernible print-out image, while avoiding any further loss in speed. Alternatively, improvements in photo-efficiency make possible further reductions in the concentration of print-out constituents so as to improve photospeed, while not detracting from the ability to discern a print-out image. Improvement in the photo-efficiency is very difficult to accomplish, since it must be achieved without deleteriously affecting the properties of the print-out composition or the radiation-sensitive polymer composition in which it is incorporated.
It is toward the objective of providing an improved lithographic printing plate comprising a photocrosslinkable polymer and a print-out composition with enhanced photo-efficiency that the present invention is directed.